<style type="text/css">
li{cursor:pointer;}
.cur{background:red;}
</style>
<script type="text/javascript">
window.onload = function ()
{
 var aLi = document.getElementsByTagName("li");
 var i = 0; 
 for (i = 0; i < aLi.length; i++)
 { 
 aLi[i].onclick = function ()
 {
  for (i = 0; i < aLi.length; i++) aLi[i].className = "";
  this.className = "cur";
 };
 }
};
</script>
<div class="clMenu">
  <ul>
    <li>1</li>
    <li>2</li>
    <li>4</li>
    <li>5</li>
  </ul>
</div>